The cellular mobile communication has evolved from the UMTS (Universal Mobile Telecommunication System) into the LTE (Long Term Evolution). In the LTE, the system based on the OFDM (Orthogonal Frequency Division Multiplexing) is specified as the radio access technology and high-speed radio packet communication with a downlink peak transmission rate of 100 Mb/s or more and an uplink peak transmission rate of 50 Mb/s or more is enabled.
The 3GPP (3rd Generation Partnership Project), which is an international standardization organization, has currently started examinations of the LTE-A (LTE-Advanced) based on the LTE toward realization of higher-speed communication. The LTE-A aims at a downlink peak transmission rate of 1 Gb/s and an uplink peak transmission rate of 500 Mb/s and various new technologies, such as the radio access system and the network architecture, are under study (for example, see 3GPP TR36.913 V9.0.0, 3GPP TR36.912 V9.3.0, and 3GPP TS36.300 V10.4.0). On the other hand, the LTE-A is the system based on the LTE, and therefore, it is important to maintain backward compatibility.
In the LTE or LTE-A, as the operation in the idle mode of a radio terminal, cell selection is specified. Specifically, cell selection and cell reselection are specified (for example, see 3GPP TS36.304 V10.2.0 and 3GPP TS36.133 V10.3.0).
The cell selection is performed when the radio terminal turns on the power and a PLMN (Public Land Mobile Network: mobile network operator) is selected. As the cell selection, cell selection (initial cell selection) performed by a radio terminal without knowing information of a cell and cell selection (stored information cell selection) performed by a mobile station with knowledge of information of a cell are specified.
In the cell selection, a radio terminal measures radio quality and selects a cell of good radio quality as a serving cell and camps on a network. More specifically, if the cell selection criteria “S” determined by RSRP (Reference Signal Received Power) and RSRQ (Reference Signal Received Quality) are satisfied, it is possible to camp on the cell (for example, see 3GPP TS36.304 V10.2.0). The radio terminal may receive an incoming call by camping on the network. The cell reselection is performed in order to detect a cell of better radio quality and when better radio quality is detected, the radio terminal camps on the cell.
Radio measurement in the idle mode is specified in order to detect a cell of better radio quality (for example, see 3GPP TS36.133 V10.3.0). In the idle mode, it is needed to achieve a balance between power consumption of the radio terminal and accuracy of radio measurement.
For example, if the frequency of measurement is reduced in order to suppress power consumption, the accuracy of measurement deteriorates and there occurs a case where it is not possible to camp on an appropriate cell. On the other hand, if the frequency of measurement is increased in order to improve the accuracy of measurement, power consumption increases. In view of this point, DRX (Discontinuous Reception) is specified (for example, see 3GPP TS36.133 V10.3.0).
There are cases where a DRX cycle value of the DRX is acquired by broadcast information broadcasted by the base station and where it is set by the NAS (Non Access Stratum), which is an upper layer. The radio terminal performs measurement at least once for each DRX and samples the radio quality. The radio terminal then averages the radio quality according to sampling intervals specified by the function of the DRX and then calculates a measured value of the radio quality.
Further, the radio terminal in the idle mode periodically monitors a paging signal in order to detect an incoming call. In the radio terminal, as in the case of the measurement described above, if the frequency of monitoring of a paging signal is reduced, a communication delay occurs and if the frequency of monitoring of a paging signal is increased, power consumption increases. Therefore, it is specified that monitoring of a paging signal is performed only once within the DRX cycle (for example, see 3GPP TS36.304 V10.2.0).
As described above, it is possible for the radio terminal to perform cell selection and incoming call detection in consideration of power consumption by performing measurement and monitoring of a paging signal within the DRX cycle, which is the cycle of measurement.
It is specified that decision of whether or not to perform measurement and cell selection is performed at least once for each DRX cycle. Further, it is specified that sample values (specifically, values of RSRP and RSRQ) of the radio quality obtained by measurement are filtered and averaged where the sample values are spaced by at least half of the DRX duration when calculating a measured value of measurement (for example, see 3GPP TS36.133 V10.3.0).
Therefore, if the DRX cycle is increased in order to suppress power consumption of the radio terminal, the sampling interval of measurement increases and there has been such a problem that the accuracy of measurement deteriorates.